1. Field of the Invention
The present invention relates to determining an impulse response as well as to presenting an audio piece in an environment of which an impulse response has been determined.
2. Description of the Related Art
There is an increasing need for new technologies and innovative products in the area of entertainment electronics. It is an important prerequisite for the success of new multimedia systems to offer optimal functionalities or capabilities. This is achieved by the employment of digital technologies and, in particular, computer technology. Examples for this are the applications offering an enhanced close-to-reality audiovisual impression. In previous audio systems, a substantial disadvantage lies in the quality of the spatial sound reproduction of natural, but also of virtual environments.
Methods of multi-channel speaker reproduction of audio signals have been known and standardized for many years. All usual techniques have the disadvantage that both the site of the speakers and the position of the listener are already impressed on the transfer format. With wrong arrangement of the speakers with reference to the listener, the audio quality suffers significantly. Optimal sound is only possible in a small area of the reproduction space, the so-called sweet spot.
A better natural spatial impression as well as greater enclosure or envelope in the audio reproduction may be achieved with the aid of a new technology. The principles of this technology, the so-called wave-field synthesis (WFS), have been studied at the TU Delft and first presented in the late 80s (Berkout, A. J.; de Vries, D.; Vogel, P.: Acoustic control by Wave-field Synthesis. JASA 93, 993).
Due to this method's enormous requirements for computer power and transfer rates, the wave-field synthesis has up to now only rarely been employed in practice. Only the progress in the area of the microprocessor technology and the audio encoding do permit the employment of this technology in concrete applications today. First products in the professional area are expected next year. In a few years, first wave-field synthesis applications for the consumer area are also supposed to come on the market.
The basic idea of WFS is based on the application of Huygens' principle of the wave theory:
Each point caught by a wave is starting point of an elementary wave propagating in spherical or circular manner.
Applied on acoustics, every arbitrary shape of an incoming wave front may be replicated by a large amount of speakers arranged next to each other (a so called speaker array). In the simplest case, a single point source to be reproduced and a linear arrangement of the speakers, the audio signals of each speaker have to be fed with a time delay and amplitude scaling so that the radiating sound fields of the individual speakers overlay correctly. With several sound sources, for each source the contribution to each speaker is calculated separately and the resulting signals are added. If the sources to be reproduced are in a room with reflecting walls, reflections also have to be reproduced via the speaker array as additional sources. Thus, the expenditure in the calculation strongly depends on the number of sound sources, the reflection properties of the recording room, and the number of speakers.
In particular, the advantage of this technique is that a natural spatial sound impression across a great area of the reproduction space is possible. In contrast to the known techniques, direction and distance of sound sources are reproduced in a very exact manner. To a limited degree, virtual sound sources may even be positioned between the real speaker array and the listener.
Although the wave-field synthesis functions well for environments whose properties are known, irregularities occur if the property changes or the wave-field synthesis is executed on the basis of an environment property not matching the actual property of the environment.
An environment property may be described by the impulse response of the environment.
This is set forth in greater detail on the basis of the subsequent example. It is being started from the fact that a speaker sends out a sound signal against a wall the reflection of which is undesired. For this simple example, the space compensation using the wave-field synthesis would be to at first determine the reflection of this wall in order to determine when a sound signal having been reflected from the wall arrives again at the speaker, and which amplitude this reflected sound signal has. If the reflection from this wall is undesirable, there is the possibility with the wave-field synthesis to eliminate the reflection from this wall by impressing a signal of opposite phase regarding the reflection signal with corresponding amplitude in addition to the original audio signal on the speaker, so that the outbound compensation wave extinguishes the reflection wave, such that the reflection from this wall is eliminated in the environment being considered. This may take place by at first calculating the impulse response of the environment and determining the property and position of the wall on the basis of the impulse response of this environment, with the wall being interpreted as mirror source, i.e. as sound source, reflecting incident sound.
If at first the impulse response of this environment is measured and then the compensation signal which has to be impressed on the speaker superimposed on the audio signal is calculated, cancellation of the reflection from this wall will take place, such that a listener in this environment sonically has the impression that this wall does not exist at all.
It is, however, critical for optimum compensation of the reflected wave that the impulse response of the room is determined accurately so that no over- or undercompensation occurs.
In a presentation room there is a problem in that it is almost impossible to measure the real impulse response of an environment, since in a presentation room, such as a movie theater, a concert hall, or also the living room at home, constant changes of the environment take place. In other words, in a movie theater presentation room it cannot be predicted how many people come to a certain presentation. If for the wave field synthesis an impulse response optimally calculated for an empty presentation room was employed, wherein in the calculation of the impulse response no people were in the room, overcompensation of the reflected sound wave would take place due to the attenuation of people present at the presentation, in that two disadvantages arise. On the one hand, the reflection at the wall is no longer optimally compensated for. On the other hand, due to the overcompensation, since the attenuation of the reflected wave by the impulse response underlying the wave-field synthesis is no longer sensed optimally, an additional audible spurious signal detracting from the overall audio impression will occur.
Optimum application of the wave-field synthesis depends on the environment in which it is being presented always being optimally sensed in order to achieve desired aims, such as special acoustics, or not to introduce audible interferences.
One possibility would be to fit a concert hall, for example, with dummy audience the reflection properties of which correspond to those of living audience. Then, a corresponding impulse response could be determined, which corresponds to the real situation at least better than when using the impulse response of the empty concert hall, i.e. without any audience, for wave-field synthesis.
This procedure is disadvantageous in that in a public presentation, just like e.g. in the living room at home, it cannot be predicted how many audience come to the presentation. An optimum sound impression is then only achieved when the number of dummy audience and the positioning of the dummy audience almost correspond to the actual number and positioning of the living audience. Moreover, the expenditure for fitting a major movie theater or concert hall with a lot of dummy audience is considerable.
Alternatives to the determination of a real impulse response are to measure the impulse response of the room shortly before the beginning of the presentation, i.e. when the presentation room is already filled with the audience actually going to be present at the presentation, in order to have a realistic description of environment, which will only strongly deviate from the actual situation if for example after a break a lot of audience would no longer be present at the presentation, etc.
This procedure, however, is problematic from two aspects. On the one hand, the calculation of the impulse response of a room takes a certain time. On the other hand, the determination has to take place immediately prior to the beginning of the presentation so that, if possible, all audience already are in the presentation room. Since it is exactly the presence of the audience that is critical, it is not avoidable in this procedure that the audience all have to wait until the measurement is completed, so that in this procedure the actual beginning of the presentation would always be postponed. When becoming known among the audience, this procedure would lead to the fact that most of the audience would only come later than at the actual beginning of the presentation, so that the actual aim, i.e. to sense an impulse response of an environment in realistic surroundings, again cannot be achieved.
Moreover, it is problematic that, for impulse response determination in a presentation room, acoustic signals have to be fed into the room, and that these acoustic signals should have considerable energy in particular in larger presentation rooms, in order to achieve secure impulse response determination. Experiments with acoustic chirps prior to the beginning of the presentation for the determination of the impulse response, i.e. as measuring signals sent out via speakers, have shown that this method is not particularly feasible. On the one hand, many listeners found the acoustic chirps sent out with considerable volume annoying. Other audience began to imitate the chirps from the speaker themselves so that measurement of the reaction signal to the acoustic chirps was problematic to impossible, since it could not be discriminated whether the chirps come from the speaker or whether it was chirps imitated by people.
Alternative procedures for the determination of the impulse response of a room are to use a pseudonoise sequence with a white spectrum as measuring signal. Although the noise cannot immediately be imitated by the audience, it is still annoying for many people and, when this method would be applied again and again, lead to the fact that the people would no longer come to the beginning of the presentation as indicated, but only a certain amount of time later, when they can safely assume that the impulse response determination of the presentation room perceived as annoying is already completed.